1. Field of the Invention
The present invention relates to electric power supply systems and, more particularly, to an electric power supply system which includes a commercial power source and another power source and in which power is supplied to load by employing those two power sources in combination.
2. Description of the Background Art
Conventionally, non-utility generators or the like have been used in place of commercial electric power in factories or the like in order to reduce concentrated demands for commercial electric power due to a concentrated use of coolers in the daytime, especially the summer time. Recently, solar photovoltaic cells which have been recently developed have also been used as one of countermeasures for the reduction of concentrated demands for commercial power. Further, a system has been developed in which a solar photovoltaic cell and a commercial power source are used in combination by effectively using characteristics of the solar photovoltaic cell that generates power only in the daytime.
FIG. 13 is a schematic diagram showing structure of a conventional typical solar photovoltaic cell utilizing system in combination with a commercial power source.
With reference to FIG. 13, this system includes a solar photovoltaic array 1, a commercial power source 4, a charge/discharge control unit 30, a storage battery 31, a DC/AC inverter 32 for converting an applied direct current (DC) voltage into an alternating current (AC) voltage and outputting the converted voltage, a switch 33, an air conditioner 34 being the load of the system, as well as a lighting unit 35, a refrigerator 36 and the like.
A DC voltage output from solar photovoltaic array 1 is applied via charge/discharge control unit 30 and storage battery 31 to DC/AC inverter 32. The applied voltage is then converted into an AC voltage of a predetermined level by DC/AC inverter 32 and then applied to switch 33 at a succeeding stage. Switch 33 is also supplied with a voltage of 100 V (AC) from commercial power source 4 connected on the other side. Switch 33 has its signal input changed over to either DC/AC inverter 32 or commercial power source 4 and operates to provide a voltage applied to the changed-over input to its output. Accordingly, load registered in the system is supplied with power necessary for driving of the load from either the solar photovoltaic array 1 or the commercial power source 4. Switch 33 is provided to avoid a mixture of AC voltages.
Charge/discharge control unit 30 and storage battery 31 are provided in association with solar photovoltaic array 1 to inhibit output variations of solar photovoltaic array 1. More specifically, when an output of solar photovoltaic array 1 is larger than the power required by load because of sufficient insolation, the output of solar photovoltaic array 1 is supplied to the load and excess power is stored in storage battery 31. Conversely, in cloudy days or rainy days, since power is insufficient to drive the load by utilizing only the output of solar photovoltaic array 1, the deficiency in power is replenished by storage battery 31 through charge/discharge control unit 30. Accordingly, charge/discharge control unit 30 and storage battery 31 serve to inhibit output variations of solar photovoltaic array 1 in accordance with sunshine conditions, so as to obtain sufficient power to stably drive the load. If the above-described commercial power source-combined type solar photovoltaic cell utilizing system of FIG. 13 is adopted, the concentrated demands for power with respect to commercial power source 4 can be reduced.
However, since the aforementioned solar photovoltaic cell utilizing system shown in FIG. 13 requires charge/discharge control unit 30 and storage battery 31 in association with solar photovoltaic array 1, the initial cost for provision of the system is increased. This results in a problem that it is difficult to make the system come into wide use.
FIG. 14 is a diagram for use in explaining a procedure of energy conversion in a case where power is supplied to an inverter air conditioner in the commercial power source-combined type solar photovoltaic cell utilizing system shown in FIG. 13.
There has been a further disadvantage that a utilization efficiency of energy is low in a case where air conditioner 34 is driven by utilizing solar photovoltaic array 1 as shown in FIG. 14. That is to say, air conditioner 34 includes a compressor 7. Since air conditioner 34 drives compressor 7 to circulate a refrigerant for heat exchange, a voltage multiplying rectifier circuit 5 and a VVVF (Variable Voltage Variable Frequency) inverter 6 are newly provided for supplying AC power for driving compressor 7 to compressor 7. Accordingly when air conditioner 34 is driven by utilizing solar photovoltaic array 1, DC power output from solar photovoltaic array 1 is once converted into the same AC power as commercial power and further undergoes electric power conversion for driving compressor 7 in air conditioner 34, resulting in a problem that energy loss cannot be avoided in double power conversion.